\$\begingroup\$@Stevensmethurst We can't always know why the voltage or current rating for a component has the value that it does. There may be some aspect of the component that the manufacturer decides will limit its working voltage even if that aspect of the component is not visible to us.\$\endgroup\$
– Elliot AldersonFeb 24 at 23:20

Also see both the useful link from Edgar and the useful links from those posts.

2233709 is "officially" correct - a switch is USUALLY marked both with ratings AND with certifying bodies who specify the applicable conditions for the test. In some cases manufacturers specify switch ratings without reference to a formal test specification.

Switch current ratings generally relate to the power dissipation in the switch when the contacts are closed so in a given switch a 10A current will typically generate a quarter of the power of a 20A current, as power is equal to current squared x resistance.
Switch Voltage ratings are generally mainly related to the ability to break an arc which occurs as switch is opened.
The difficulty in breaking an arc is much greater for DC than for AC.
Accordingly halving the current makes the load on the switch very much lower whereas doubling the voltage increases the design difficulty in closer to a linear manner at very low voltages.

In all cases, but especially for uncertified switches where the basis for the ratings are not well defined, it is likely,but not certain, that a 20 amp DC rated switch will handle 10 Amps at 20 volts “safely”. However, if it doesn't you have no come back and in situations like an aircraft supply I wouldn't consider doing so.

As a useful but not certain 'rule', switches from reputable manufacturers tend to have certified ratings which are traceable to a standards authority and switches without certification are liable to be from manufacturers who are less reputable. This does not mean that a switch without certification marks is necessarily incorrectly marked but it does suggest extra caution is required. Similarly a switch with many certification marks but which is from a manufacturer of unknown reputation makes it more possible that the certifications are not valid.

The above mealy mouthed / weasel worded pontifications are a long way of saying that “Switches with fake markings from unknown sources are not at all uncommon in the industry - “Caveat Emptor” "

In cases which are life critical I would only use switches which

had formal test specifications and certifications and

were made by reputable manufacturers and

were sourced from known good supply chains.

The importance of the last point is worth noting. If you buy eg a Sprague switch from Digikey it is probably a Sprague switch and probably it's specs can be relied on. If you buy a "Sprague" switch from Seller1234567 on ebay it MAY or may not be a Sprague switch and its specs may or may not be able to be relied on.

\$\begingroup\$Arguably any application like a fast battery charger or heater should be considered 'life-critical', because of the risk of overheating and fire if left energised when it shouldn't be. I have known cheap switches fail where the rocker moved but the switch stayed closed, so it might not be immediately obvious that the equipment was still powered.\$\endgroup\$
– nekomaticFeb 26 at 14:05

\$\begingroup\$@nekomatic Yes, and/but, often as not they use no name brand un-genuinely-certified switches with suspect if any ratings.\$\endgroup\$
– Russell McMahonFeb 27 at 6:58

The two ratings (voltage and current) for a switch are largely independant. It makes no sense to multiply them together and treat this as a parameter of the switch.

For a switch, you must comply with both of the ratings at once, you can't really say that because the current is low, the voltage rating can be exceeded.

It is true that as the switch opens and starts to draw an arc, if that arc is a very low current then it is more likely to be extinguished as the switch contacts start to open, but this really would only be a safe assumption with a current of 10s of mA (and even then, you're working outside of the specification). Failure will also not always be immediately obvious - the problem might be slow degredation which leads to a failure (or fire) after many switch operations.